The locus of electrically excitable tissue where action potentials are induced can be controlled using the physiological principle of electrotonus. In each of the three-dimensional images, pixels in a region specified by a cross section perpendicular to an observation direction and a thickness are projected, and the projection images are generated by repeating the procedure while moving a position of the cross section in the observation direction. The current sensor also includes a soft ferromagnetic body disposed within the outer body comprising a first core element and a second core element.